The penetration of toxic, antigenic or enzymatic irritants from the subgingial plaque into the tissues of the gingival crevice represents one of the most fundamental events in the initiation of inflammatory periodontal disease. Thus, the ability of the eipthelium which lines the gingival crevice to act as a effective barrier is an important mechanism of host defense. The primary objectives of this investigation are to quantitatively evaluate the permeability of gingival crevicular epithelium during states of gingival health, healing or inflammation, and to correlate ultrastructural morphometric changes in the epithelium with changes in permeability. In addition, the ability of several agents including urea, 1,3 butanediol, cetylpyridinium chloride, tannic acid, polyphloretin phosphate, mineral oil and gastric mucin, to improve the effectiveness of the crevicular epithelial barrier will be determined. Finally, the clinical usefulness of the above barrier agents in preventing or delaying the onset of gingivitis in an experimental animal model system will be determined.